Mixing apparatus, as previously devised, comprised a supply line for each of the components and a fluid-flow measuring device connected to the supply line for the first component and capable of generating a number of electrical pulses corresponding to the flow volume. A feed pump connected to the supply line for each additional component had an electric driving motor for each feed pump, with a current supply circuit for each driving motor for maintaining an adjusted mixing ratio and to which the pulses generated by the fluid-flow measuring device were fed.
In a known mixing apparatus of the above described type, which was used as a dosing system for a two-component lacquer whose one component was the stock lacquer and the other a hardener, the fluid-flow measuring device was formed by a heat pulse emitter/receiver. With this structural element, heat pulses were supplied to the flowing stock lacquer component at the start of a measuring section, the arrival of the heat pulses at the end of the section being established. The transit time of the heat pulses was a measure for the flow volume. The frequency of a pulse generator was controlled in correspondence with said transit time. The pulses emitted by the pulse generator were supplied to an electronic control circuit as the set value.
As the actual value, the electronic regulator circuit received pulses from a fluid-flow measuring device which measured the flow volume of the hardener component and also operated according to the principle of the heat pulse emitter/receiver. The electronic regulator circuit generated a continuous regulating signal which was fed to a servomotor that drove a geared pump connected to the hardener conduit. The fluid-flow measuring device which measured the flow of the hardener component was connected to the output of the geared pump. The outputs of the two fluid-flow measuring devices were connected with a mixing and spraying unit and the mixing ratio was adjustable on the electronic regulating circuit.
The known mixing apparatus showed poor dynamic performance in the turn-on phase and in the turn-off phase. This was manifested by the adjusted mixing ratio remaining constant only a few seconds after turn-on. Therefore, the constancy of the mixing ratio of the lacquer which continued spraying during a short time after turn-off was not assured. Tests showed that this poor dynamic performance originated in the sluggishness of the heat pulse emitter/receiver used as the fluid-flow measuring device as well as on the natural transient response of the analogously operated regulator circuit and of the servomotor.
One general object of the invention is therefore to provide a mixing apparatus of the type described with improved dynamic performance.
Another object of the invention is to provide an improved mixing apparatus which excels in particular by the adjusted mixing ratio remaining constant immediately after turn-on.